1. Field of the Invention
The present invention generally relates to antenna structures and, more particularly, is concerned with a slot antenna backed by an electrically small cavity and tuned to resonance primarily by dielectric loading placed over the slot and secondarily by variable capacitance played symmetrically across the slot which combination results in reduction of the physical size and increase in the gain and efficiency of the antenna.
2. Description of the Prior Art
Cavity-backed slot antennas are well known in the prior art. Traditionally, they are composed of a metal surface backed by an energized resonant cavity and having a slot through which energy is radiated directionally. Representative of the prior art are the cavity-backed slot antennas disclosed in U.S. Pats. Nos. Lindenblad (2,570,824), Fales (2,684,444), Turner (2,863,145), Baldwin (2,885,676), Charman (3,056,130), Harris et al (3,550,141), Monser (4,132,995), Sanford (4,242,685) and Schiavone (4,367,475). As recognized in most of these patents, radiating slot antennas are particularly useful in applications where the antenna must conform to an external surface so as to not interfere with the desired characteristics of the surface. For example, a cavity-backed slot antenna is advantageously used in conjunction with an aircraft wing or fuselage since it will not adversely affect the aerodynamics of the aircraft surface.
The resonant cavity which backs the radiating slot is typically provided on the interior side of the aerodynamic surface in order to limit radiation of energy to the exterior side thereof. To accommodate the cavity there must be unused space available. But, in most applications, interior space is at a premium. Therefore, one basic objective in cavity-backed slot antenna design must be to minimize the physical size of the cavity to the extent feasible without unduly sacrificing the performance characteristics of the antenna.
Various approaches have been proposed in certain of the above-cited patents to limit the physical size of the cavity-backed slot antenna. For instance, bending of the resonant cavity in Fales (U.S. Pat. No. 2,684,444) is proposed to reduce the space occupied by the cavity. In Charman (U.S. Pat. No. 3,056,130), a cavity of a size smaller than normally required for resonance at a given desired frequency is provided. The cavity is tuned to desired resonance by capacitive loading via a baffle located along a longitudinal axis within the cavity. Also, Charman suggests that a small variable capacitance may be provided between the baffle and the bottom of the cavity for the purpose of practical adjustment of the resonant frequency. In Sanford (U.S. Pat. No. 4,242,685), an electrically conductive plate is disposed within the cavity and spaced from all of its internal walls so as to lengthen the effective electrical resonant dimensions of the cavity for a given physical size. The resonant cavity can thus be smaller in size for a given frequency of operation.
The cavity-backed slot antennas of the cited prior art which are identified above as ones concerned with space conversation would appear to operate resonably well and generally achieve their objectives under the range of operating conditions for which they were designed. However, they do provide opportunities for further improvements to be made in terms of reduction of the complexity and constraints they introduced into their antenna designs to achieve the objective of reduced size. Consequently, a need still exists for improvements in cavity-backed slot antenna design which will make size reduction possible without introducing other factors which will diminish antenna performance and increase complexity and cost.